How do I preview in composing?
The main window is the Compositor, and not visible in the screenshot there must be a Viewer Node to the right. Backdrop is disabled, so you don't see the composite behind the nodes.
In the bottom left corner is the UV/Image Editor and below your red frame you can see that the display is set to Viewer Node instead of the default Render Result.
So everything changing in the Compositor which you normally watch in the backdrop is displayed in the Image Editor.